S. Jay Chey

Control of an interfacial MoSe2 layer in Cu2ZnSnSe4 thin film solar cells: 8.9% power conversion efficiency with a TiN diffusion barrier

We have examined Cu2ZnSnSe4 (CZTSe) solar cells prepared by thermal co-evaporation on Mo-coated glass substrates followed by post-deposition annealing under Se ambient. We show that the control of an interfacial MoSe2 layer thickness and the introduction of an adequate Se partial pressure (PSe) during annealing are essential to achieve high efficiency CZTSe solar cells—a reverse correlation between device performance and MoSe2 thickness is observed, and insufficient PSe leads to the formation of defects within the bandgap as revealed by photoluminescence measurements. Using a TiN diffusion barrier, we demonstrate 8.9% efficiency CZTSe devices with a long lifetime of photo-generated carriers.

Torwards marketable efficiency solution-processed kesterite and chalcopyrite photovoltaic devices

Although CuIn1−xGaxSe2−ySy (CIGS) chalcopyrite and Cu2ZnSn(S,Se)4 (CZTSSe) kesterite-related films offer significant potential for low-cost high-efficiency photovoltaic (PV) devices, the complicated multi-element nature of these materials generally leads to the requirement of more complex and costly deposition processes. This talk focuses on employing the unique solvent properties of hydrazine to solution-deposit CIGS and CZTSSe films for high-performance solar cells. CIGS films are deposited by completely dissolving all elements in hydrazine, solution-depositing a molecular precursor film, and heat treating in an inert atmosphere, to yield a single-phase chalcopyrite film (no post-deposition selenization required). Trace additions of Sb improve grain structure in the resulting film and enhance device performance. Devices based on a glass/Mo/spin-coated CIGS/CdS/i-ZnO/ITO structure yield power conversion efficiencies of as high as 13.6% (AM1.5 illumination; NREL certified). Analogous CZTSSe absorber layers have been processed using a hybrid hydrazine-based slurry approach, enabling liquid-based deposition of kesterite-type films and resulting device efficiencies of as high as 9.6% (AM1.5 illumination; NREL certified)—exceeding the previous kesterite performance record by ∼40%. The combination of improved efficiency, In-free absorber and solution-based processing opens opportunities for development of a low-cost and pervasive technology.

Manipulation and writing with Ag nanocrystals on Si(111)-7×7

Manipulation of Ag nanocrystals derived from up to 200 000 atoms was performed using the tip of a scanning tunneling microscope. By varying the scanning conditions, it was possible to move them laterally on contamination-free Si(111)-7×7 surfaces or to remove them. In both cases, thin Ag tracks were left behind. This demonstrates the concepts of nano patterning and nano painting with metals on clean semiconductor surfaces.

High efficiency Cu 2 ZnSnSe 4 solar cells with a TiN diffusion barrier on the molybdenum bottom contact

We report on the structural properties and device results of Cu₂ZnSnSe₄ (CZTSe) solar cells deposited using a vacuum deposition process on glass substrates coated with a molybdenum bottom electrode. Compared to similarly-prepared pure sulfide Cu₂ZnSnS₄ (CZTS) devices, CZTSe devices exhibit a much thicker interfacial MoSe₂ reaction layer. This poses a serious problem in achieving high efficiency CZTSe solar cells-an overall reverse correlation between device performance and MoSe₂ thickness is observed. We show that the interfacial MoSe₂ formation can be controlled by the use of thin TiN diffusion barriers. Using this process we demonstrate a CZTSe device with 8.9% efficiency.

Solution processing of CIGS absorber layers using a hydrazine-based approach

A simple solution-based approach has been developed for CIGS absorber layer deposition, employing hydrazine as the solvent for all metal chalcogenide components. Advantages of the technique include the molecular (rather than nano or microparticle) nature of the precursor solutions, which enables intimate mixing of the various CIGS components before the final heat treatment, the absence of carbon, oxygen and other common contaminants from the solution, and the lack of need for a post deposition selenization treatment to achieve highquality CIGS films. Relatively smooth and compact films, with up to μm-scaled thicknesses and grain sizes, have been achieved by spin coating. Gallium and sulfur have been successfully incorporated into the Cu1−zIn1−xGaxSe2−ySy layers for x≪0.5, y≪0.6 and z≪0.15. Preliminary PV devices based on a glass/Mo/CIGS/CdS/i-ZnO/ITO structure and employing the solution-processed CIGS films have yielded efficiencies of up to 10% (AM 1.5 illumination).

Characterization of indium tin oxide and al-doped Zinc oxide thin films deposited by confocal RF magnetron sputter deposition

Thin film properties (resistivity, sheet resistance, optical transmissivity, stability testing under RH85/85C conditions and film stress) were measured for indium tin oxide (ITO) and Al-doped Zinc Oxide (ZnO) (2 wt.% Al doped target) films deposited using a confocal RF magnetron sputtering system. A comparison was made between sample biasing and high temperature conditions with respect to these properties. The sample bias was applied by RF power (0 – 60 W) to the sample. For the high temperature runs, the samples were heated to temperatures of as high as 250 C. ITO was deposited with argon as the process gas; Al-ZnO was deposited with a small amount of hydrogen (from 0 to 1%) added to argon. We find that comparable qualities of films can be obtained by either sample biasing or high temperature processes for ITO and Al-ZnO in terms of sheet resistance and transmission. However, sample biasing resulted in significantly higher compressive stress. The sheet resistance of Al-ZnO was affected by addition of hydrogen. The optimal concentration of hydrogen was 0.33% for sample biasing and 0.5% for high temperature runs under the deposition conditions considered.

High efficiency Cu 2 ZnSn(S x Se 1−x ) 4 thin film solar cells by thermal co-evaporation

We report on the device results of thermally evaporated high efficiency Cu2ZnSn(SxSe1−x)4 (CZTSSe) thin film solar cells with power conversion efficiencies of 7.1% (x=1.0) and 7.5% (x=0.34). We have carried out extensive electrical and structural characterization of CZTSSe solar cells to identify major factors that limit the efficiency. Bias-dependent quantum efficiency measurements revealed ineffective collection of charge carriers photo-generated deep in the absorber layer suggesting a short minority carrier diffusion length, which was confirmed by time-resolved photoluminescence measurements. Temperature-dependence of the series resistance of the devices is consistent with the presence of a Schottky-type barrier in the back contact, likely caused by secondary phases near the CZTS/Mo interface and/or an interfacial MoSx layer.

CuIn(Se,S) 2 Absorbers Processed using a Hydrazine-Based Solution Approach

With tunable bandgap and demonstrated high efficiency, the chalcopyrite CuInSe2 and its alloys have shown great potential as absorbers for single and multi-junction solar cells. However, the current deposition techniques mostly rely on expensive vacuum-based processing or involve complicated precursor solution preparation. These higher-cost absorber preparation processes make it difficult to commercialize this technology. In this work, CuInSe2-xSx (CIS) absorbers are deposited using a simple hydrazine-based solution process. Precursor solutions were prepared by dissolving the component metal chalcogenides and chalcogen in hydrazine, forming homogeneous solutions containing adjustable concentrations of desired elements mixed on a molecular level. These precursor solutions are then spin coated on substrates followed by a heat treatment in an inert environment to produce high quality CIS thin films. Significantly, no post deposition selenization process is required using this technique. Laboratory scale devices with conventional glass/Mo/CIS/CdS/i-ZnO/ITO structure have been fabricated using CIS absorbers deposited via this process. For the baseline low-bandgap CIS system with no Ga added (to compare with our previously reported results with Ga incorporated), AM1.5G conversion efficiency of as high as ~9% has been achieved for devices with 0.45 cm 2 effective area.

Novel Si-based composite thin films for 193/157-nm attenuated phase-shift mask (APSM) applications

We have developed a novel Si-based composite thin film for attenuated phase shift mask(APSM) applications at 193/157 nm wavelength. The fabrication involved sputtering deposition, either with dual target or a single composite target. At 193 nm, these thin films show tunable optical transmission and good stability against long term radiation, common chemicals used to strip photoresist, and exhibit good dry etch selectivity to quartz. Specifically, a film with initial transmission of 5.72%,the total increase oftransmission was 0.27% for doses up to 5.4 kJ/cm2. Also, the increase of transmission was 0.19% after 60 mm of cleaning treatment in acid based solution (H2S04H20210:1 at 95°C). The dry etch selectivity over fused quartz was greater than 5:1. The transmission of the films at 193 nm can be tuned from 0 % to 20 % by varying the thin film composition, process gas flow and composition, and deposition pressure. This wide transmission window provides the possible extension down to 157 nm wavelength.

Standing up to be counted

The only major player willing to offer some thoughts in the chalcogenide sector has been IBM, whose Drs David Mitzi, Mathew Copel and S Jay Chey published a paper in Advanced Materials 2005 17, 1285-1289, on a low voltage transistor, produced using a high mobility spin-coated chalcogenide.